A pressure sensor for detecting pressure by diaphragms constructed of trenches formed in a semiconductor substrate has been proposed by, for example, patent documents 1 and 2.
Specifically, in the patent document 1, a first trench having a specified depth is formed in an N-type substrate and a second trench having a specified depth is formed in such a way as to surround the first trench. In this way, a wall between the first trench and the second trench is constructed as a diaphragm. Further, in order to prevent portions opposite to each other in the diaphragms from being electrically connected to each other, P-type impurities are introduced into the diaphragms. Further, the upper portion of the first trench is covered with an insulating film and hence the interior of the first trench is hermetically sealed, whereby a reference pressure chamber is constructed.
According to this construction, the diaphragm is deformed in response to a pressure difference between the inside and the outside of the first trench, so that a distance between the p-type impurity regions of the respective diaphragms is varied and hence a capacitance between the p-type impurity regions of the respective diaphragms is varied. Hence, when a variation in the capacitance between the p-type impurity regions is taken out, the pressure can be detected.
Further, in the patent document 2, for example, an N-type semiconductor thin film is formed over a support substrate via an insulating separation film, and a first trench and a second trench are formed in this semiconductor thin film in such a way as to be parallel to each other. Further, of the semiconductor thin film, a portion sandwiched by the respective trenches is constructed as a diaphragm which can be elastically deformed. This diaphragm has, for example, P-type impurities introduced therein at a high concentration. Further, the semiconductor thin film has a fixed electrode part formed therein, the fixed electrode part having P-type impurities introduced into a position corresponding to the diaphragm via the first trench at a high concentration. The first trench is hermetically sealed by a sealing film, and this hermetically sealed space is made a pressure reference chamber.
According to this construction, a capacitor in which capacitance is varied in response to the deformation of the diaphragm is formed between the diaphragm and the fixed electrode. Hence, by taking out a variation in the capacitance, the pressure can be detected.    [Patent document 1] JP-A-S63-175737    [Patent document 2] JP-A-H11-220137
However, in the patent document 1, the P-type impurity region for detecting the capacitance is formed in the N-type diaphragm, so that a PN junction part is formed between the N-type diaphragm and the P-type impurity region. Similarly, in the patent document 2, the P-type fixed electrode part is formed in the N-type semiconductor thin film, so that a PN junction part is formed between the fixed electrode part and the semiconductor thin film.
In this way, in the related art described above, the electrode for detecting the capacitance is formed of a separation structure by the PN junction. For this reason, when the pressure is detected in a high temperature atmosphere, minority carriers are increased by a temperature increase and hence a leak current essential to the PN junction flows. This presents a problem that capacitance detection characteristics such as sensitivity and accuracy are made unstable depending on the temperature.
Although there has been described above the problem presented in the case of detecting the pressure, the same problem can be said for the physical quantity which can be detected by taking out a variation in the capacitance which is caused by the deformation of the diaphragm.